The heart of Nandom is a fork in the road. It is here, in one of the northernmost towns in Ghana, that the buses come and go. You would call it a station if it was anything more than a triangle of reddish dust, surrounded by fast-food stalls, general stores and the rural bank. Once a week, a market sets up. The rest of the time, it’s the buses themselves, privately owned mini-vans known as “tro tros”, daubed with prayers for the road – “Lord Have Mercy”, “My Redeemer Liveth” – that provide the action: the logic-defying piling on of people and goods, the waiting, in midday temperatures of 40°C, for enough passengers to fill a van.

The buses are important to Nandom, the administrative centre for more than 50,000 people – chiefly farmers – in one of the poorest corners of Ghana, because the population is leaving. Migration has long been part of life in the dry reaches of west Africa, but in recent years, with economic development taking place elsewhere and erratic rains making rural life increasingly difficult, more and more people are taking to the road. The figures are inexact, but about 20 per cent of those born in northern Ghana are now thought to live in the richer, more urbanised south. In Nandom, the numbers are much higher: half the population has gone. People from the town offer varying reasons for the exodus – lack of jobs, enticing “greener pastures”, deteriorating climate – but they agree that it cannot go on indefinitely, this whittling, or Nandom will never prosper. “What is happening,” a local priest told me, “is that our society is to a certain extent being disintegrated.”

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The volatility is here to stay. Climate models predict a one or two degree rise in temperature in Ghana and West Africa by mid-century and a simultaneous 10 per cent decrease in rainfall, but the line is a jagged one. Zinedeme Minia, the deputy director-general of Ghana’s Meteorological Agency, told me it was much easier to map 20- or 30-year trends than know what will happen in the shorter term. “Climate change will bring surprises,” he said. And for poor farmers who rely on rain-fed agriculture, that uncertainty is too much to bear. “You are doing a gambling game,” said Minia. “You are never sure when you are likely to get something and when you will lose completely. That is the issue.”

Farmers in Nandom spoke in alternating tones of fatigue and bloody-mindedness about their capricious rains. One day, around noon, I met Leo Yiryel, the 84-year-old chief of a small village just outside the town. “The rain is punishing us a lot,” he said. “There used to be only a hard dry season, but now there is also a rainy season that can destroy your crops.” Yiryel was sitting on a bench under a mango tree, surrounded by grandchildren who listened raptly. There seemed to be dozens of them so I asked Yiryel where their parents were. He told me he had 12 children – a 13th died young – and that 11 of them had migrated to southern Ghana. “I never expected that,” admitted Yiryel. “I thought when they were in school and in training that all the jobs and opportunities would be here, but they are not. They are all down there.”

Knight observes, correctly, that environmental degradation has the potential to be a powerful motor for migration.

The logic is powerful: 10 per cent of the world’s population would be inundated by a 1 metre rise in sea levels – possible by the end of this century – while another 30 per cent, more than 2 billion people, live in drylands, like Nandom, that are vulnerable to endemic drought. All these people, the argument goes, will have to end up somewhere.

At the some time, he points out that the concept of the climate migration, at least as a distinct category of migrants, is perhaps questionable.

Richard Black, a professor of human geography at the University of Sussex who has studied the question for the United Nations High Commission for ­Refugees (UNHCR), told me he had several problems with the idea. In the first place, he said, it has been used to raise the spectre of massive international migration, even though ­people displaced by environmental disasters overwhelmingly tend to stay within their national borders, often as close as possible to their former homes. This can still be a great strain, but it is not the same as hordes of people crossing borders. It is also, incidentally, how migration from northern Ghana is playing out. A survey of 204 families in Nandom in 2004, for instance, found not a single relative among them who had migrated outside the country.

More broadly though, Black disputes the idea that environmental migration is somehow new, or different to other kinds of migration. The decisions of every migrant, he argues, even those made under great stress, are shaped by a mixture of economic, social and cultural factors, and the environment is just one of these. By imagining that climate change has some kind of special influence over migration, Black argues that we run the risk of overlooking similarities between ­people who move for environmental reasons and those who move for political or economic reasons, and those who do not move at all.

In the case of West Africa, at least, it certainly is. On a local scale, migration from Namdon forms part of a rational adaptation to circumstances, where work in the more prosperous south allows migrants to earn money that is then put towards remittances to their relatives who are then able to sustain their home community. On a larger scale, migration in West Africa from the dry Sahel to the richer coastal regions is a well-established tradition. For example, as noted in the online text IUCN Sahel Studies 1989, migration is established with multiple motivations, whether to richer neighbours, to areas temporarily in need of workers (to harvest a crop, for instance), from one rural region to another in pursuit of exploitable territory, and finally, through urbanization. One example of this sort of dynamic is described in Wikipedia's article on seasonal migration, where the terrible poverty experienced by a Nigerien population composed mainly of agriculturalists and herders triggers a seasonal migration out of Niger during the dry season, whether to a Nigeria that is also populated by Hausa, or to French colonies once federated with Niger and which still share a common currency. Before independence, in fact, the French government observed wide-scale emigration from Niger to the future Ghana, identifying it as a way for these people to escape oppression at home for a richer country.

In theory, then, despite environmental degradation, the sorts of normal migratory movements long-established in West Africa on the national and regional scales could provide a way for communities caught by climate change to sustain themselves. This could work out for the benefit of all: for rural communities needing to survive environmental changes, for urban communities looking for workers unskilled and otherwise, for national and regional economies which could benefit from a freer supply of labour. In theory.

In actuality, large-scale migration from the Sahel to the coasts has often had unhappy results. Adama Konseiga observes in his paper "New Patterns in the Human Migration in Africa" that these migrations, initially supplying the labour forces needed for tropical agriculture in Côte d'Ivoire and Ghana and for manufacturing in Senegal, have developed into a major force for West African integration. In examining the specific cases of Burkina Faso (a migrant-sending country) and Côte d'Ivoire (a labour-receiving country), he notes that these migrant flows served the labour needs of a very labour-intensive agricultural system based on the farming of cash crops like cocoa and coffee, these migrations surviving despite restrictions eventually placed by both countries on movement, with continuing family and other arrangements anchoring these population movements. Within a generation of independence, more than a quarter of the Ivoirien population was of post-independence migrant stock. Eventually, when Côte d'Ivoire hit hard economic times in the 1980s and migrants to urban areas began to return to their rural areas, they came into conflict with immigrant farmers already established. This, as Jonathan Edelstein noted in 2003, led to catastrophe.

The economic decline, combined with the end of the Cold War, led to pressure for democratic and fiscal reforms. [Then-president Félix] Houphouët-Boigny still had enough prestige to win the first multiparty elections in 1990, but his days were numbered; the officially-88-year-old president died in office in 1993. His place was taken by the president of the national assembly, Henri Konan Bédié, a man with the full measure of Houphouët-Boigny's autocratic tendencies but none of his redeeming virtues. Unable to hold together Houphouët-Boigny's system through charisma or personal authority, he chose to do so through chauvinism.

Under Bédié, a new word entered the Ivoirian political vocabulary - ivoirité. ivoiritéwas something relatively new on the African scene - not tribalism, but nationalism. The ivoirité movement soon became explicitly nativist, and guest workers from Mali and Burkina Faso - many of whom had the same ethnic heritage as their Ivoirian neighbors and had lived in Côte d'Ivoire for decades - became its targets. Similar hostilities have since developed in other countries - for instance, President Teodoro Obiang Nguema Mbasogo of Equatorial Guinea has urged his countrymen to attack migrant workers with machetes - and there had been previous movements against Asian merchants or disliked tribes, but intra-tribal hostility on the basis of nationality was little known at the time.

Another casualty of ivoirité was relations between the Christian south and the Muslim north, many of whom had migrated to Côte d'Ivoire during colonial times. A Bédié-era constitutional provision disqualified anyone whose parents were not Ivoirian-born from running for president, which had the effect of disenfranchising one of the main opposition leaders, Alassane Ouattara. Ouattara's constituency in the north became increasingly restive as the economy grew worse and the government reneged on many of the democratic reforms of the early 1990s.

As Micah Bump noted in his 2006 survey of Ghana, instead of being a country of immigration it's increasingly becoming a country of out-migration. Looking to the neighbouring example of Côte d'Ivoire, where northern Muslims of long standing were as much victims of ivoirité as post-independence migrants, I can't help but wonder whether a Ghana that remains poor--and perhaps other countries--can avoid Côte d'Ivoire's catastrophe. If it can't, what impact will this have on West African migration systems? Only a very small minority of West African migrants, less than a tenth, are interested in immigrating to the developed world. In the event that the sort of migratory system described above doesn't come about, will this change?